Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advances in micropower impulse radar (MIR), also known as ultra-wideband (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and low cost time domain reflectometry (TDR) instruments.
In a TDR instrument, a very fast (about 1 nanosecond) electric pulse with a rise time of 500 picoseconds, or less, is propagated down a probe, that serves as a transmission line, in a vessel. The pulse is reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. These instruments are also known as guided wave radar (GWR) measurement instruments.
To implement TDR for level measurement, it is common to use a delay lock loop (DLL) to fire a transmit pulse and then to fire a delay pulse in order to control sample time relative to transmit time. Current DLL circuits are designed to generate delay pulses during half of the pulse repetition frequency (PRF) cycle. As a result, the measurement range is limited to the amount of time in half of the PRF cycle.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.